SUMMARY

This proxy statement is being furnished to the stockholders of Nabi Biopharmaceuticals (“Nabi”) in connection with the solicitation of proxies by our board of directors for use at a special meeting of stockholders to be held on [******], 2012 at [******] a.m. local time, and at any reconvened meeting following any adjournment or postponement thereof. The special meeting will be held at [*****]. The purpose of the special meeting is for our stockholders to consider and vote upon certain proposals in connection with the transaction contemplated by the Merger Implementation Agreement, dated as of April 22, 2012, as it may be amended or supplemented (the “Transaction Agreement”), by and among Nabi and Biota Holdings Limited, a Melbourne, Australia company (“Biota”), pursuant to which each outstanding ordinary share of Biota capital stock will be acquired by Nabi in exchange for newly issued shares of Nabi common stock pursuant to a scheme of arrangement under Australian corporate law, and Biota will become a wholly-owned subsidiary of Nabi (the “Transaction” or the “scheme of arrangement”).

The following summary highlights selected information in this proxy statement and may not contain all of the information that may be important to you. Accordingly, you are urged to read carefully this entire proxy statement, including the attached annexes, and the other documents to which this proxy statement refers you in order for you to fully understand the Transaction. See “Where You Can Find More Information” beginning on page 142 of this proxy statement. Each item in this summary refers to the page of this proxy statement on which that subject is discussed in more detail.

The functional currency of Nabi is the United States (“U.S.”) dollar. Unless otherwise specified, all references to “dollars,” “$,” or “U.S.$” shall mean U.S. dollars. Biota uses the Australian dollar (“A$”) as its functional currency.

The Companies

Nabi Biopharmaceuticals (see page 81)

Nabi Biopharmaceuticals, headquartered in Rockville, Maryland, is a biopharmaceutical company that has focused on the development of vaccines addressing unmet medical needs, including nicotine addiction. Its sole product currently in development is NicVAX^® (Nicotine Conjugate Vaccine), an innovative and proprietary investigational vaccine for the treatment of nicotine addiction and prevention of smoking relapse based on patented technology.

The principal trading market for Nabi’s common stock (NASDAQ: NABI) is the NASDAQ Global Select Market (“NASDAQ”). Nabi’s principal executive offices are located at 12270 Wilkins Avenue, Rockville, MD 20852. Its telephone number is (301) 770-3099. Nabi’s website is located at www.nabi.com (the contents of which are not part of this proxy statement).

Biota Holdings Limited (see page 81)

Biota Holdings Limited, headquartered in Melbourne Australia, is an anti-infective drug discovery and development company with key expertise in respiratory diseases, particularly influenza. Biota’s first revenue-producing product, zanamivir, was developed and commercialized as the first-in-class neuraminidase inhibitor (“NI”) pursuant to a research and license agreement entered into with GlaxoSmithKline (“GSK”). GSK markets the product as Relenza™. Biota and Daiichi Sankyo co-own a range of second generation influenza NI antivirals, of which the lead product, laninamivir, is marketed in Japan by Daiichi Sankyo as Inavir^®. Biota holds a contract from the U.S. Office of Biomedical Advanced Research and Development Authority (“BARDA”) for the late stage development of laninamivir.

In addition, Biota has undertaken a Phase IIb study with its lead antiviral compound for human rhinovirus (“HRV”) infection in patients with asthma. Further Biota research achievements include a series of candidate drugs aimed at treatment of respiratory syncytial virus (“RSV”) infection and hepatitis C virus (“HCV”) infection.

The principal trading market for Biota’s ordinary shares (ASX: BTA) is the Australian Securities Exchange (“ASX”). Biota’s ordinary shares also are traded in the U.S. in the form of American Depositary Receipts (ADR Trading Symbol: BTAHY). Biota’s principal executive offices are located at 10/585 Blackburn Road, Notting Hill Victoria 3168, Australia. Its telephone number is +61 3 9915 3700. Biota’s website is located at www.biota.com.au (the contents of which are not part of this proxy statement).

Special Meeting of Nabi Stockholders

The Special Meeting (see page 45)

Nabi’s stockholders are being asked to consider and vote upon the following proposals in connection with the Transaction:

Proposals 1, 2 and 3 are collectively referred to herein as the “Charter Amendment Proposals.” Proposals 1, 2 and 4 are collectively referred to herein as the “Transaction Proposals.”

The Nabi stockholder’s vote on such proposals will take place at a special meeting to be held at [*****] a.m. local time on [*****], 2012, at [*****].

Record Date for the Special Meeting (see page 45)

You can vote at the special meeting all of the shares of Nabi’s common stock you held of record as of the close of business on [*****], 2012, which is the record date for the special meeting. As of the close of business on the record date, there were [*****] shares of Nabi’s common stock outstanding.

Recommendation of the Nabi Board of Directors (see page 61)

Nabi’s board of directors unanimously recommends that you vote “FOR” each of the proposals to be considered and voted upon at the special meeting. In connection with its decision to recommend that you vote

“FOR” each of the proposals, Nabi’s board of directors has determined that it is advisable and in the best interests of Nabi and its stockholders to adopt each amendment to the Nabi certificate of incorporation and to issue the Nabi shares in connection with the Transaction. See “The Transaction—Nabi Reasons for the Transaction” beginning on page 57 of this proxy statement and “The Transaction—Recommendation of the Nabi Board of Directors” beginning on page 61 of this proxy statement for more information about the factors considered by Nabi’s board of directors.

Required Vote (see page 48)

Each share of Nabi’s common stock is entitled to one vote at the special meeting. The holders of issued and outstanding shares of Nabi’s common stock which represent a majority of the votes entitled to be cast at the special meeting, present in person or represented by proxy, will constitute a quorum for the transaction of business at the special meeting. Abstentions will be counted for purposes of determining the presence of a quorum at the special meeting. Banks, brokers and other nominees that hold their customers’ shares in street name may not vote their customers’ shares on “non-routine” matters without instructions from their customers. As each of the proposals to be voted upon at the special meeting is considered “non-routine,” such organizations do not have discretion to vote on any proposal for which they do not receive instructions from their customers (this is referred to in this context as a “broker non-vote”). As a result, if you fail to provide your broker, bank or other nominee with any instructions regarding how to vote your shares, your shares will not be considered present at the special meeting, will not be counted for purposes of determining the presence of a quorum and will not be voted on any of the proposals. If you provide instructions to your broker, bank or other nominee which indicate how to vote your shares with respect to certain proposals but not with respect to a particular proposal, your shares will be considered present at the special meeting and be counted for purposes of determining the presence of a quorum but will not be voted with respect to that particular proposal.

Approval of the proposals presented at the special meeting will require the following:

Security Ownership of Certain Beneficial Owners and Management of Nabi Stock (see page 47)

As of the close of business on May 15, 2012, the current directors and executive officers of Nabi were deemed to beneficially own 2,722,663 shares of Nabi’s common stock, which represented approximately 6.3% of

the shares of Nabi’s common stock outstanding on that date. Beneficial ownership is determined in accordance with the rules of the Securities and Exchange Commission (“SEC”), as described below under “The Special Meeting—Security Ownership of Certain Beneficial Owners and Management of Nabi Stock” beginning on page 47 of this proxy statement.

The Transaction

Structure of the Transaction (see page 53)

On April 22, 2012, Nabi and Biota entered into the Transaction Agreement, pursuant to which Nabi will acquire all of the outstanding ordinary shares of Biota in accordance with a scheme of arrangement under Australian corporate law and to be submitted for approval by the Supreme Court of Victoria, Australia. Subject to the terms and conditions of the Transaction Agreement, upon the completion of the Transaction, each ordinary share of Biota outstanding immediately prior to the completion of the Transaction will be transferred to Nabi in exchange for 0.669212231 newly issued shares of Nabi common stock (the “exchange ratio”), subject to certain adjustments as described below. Biota does not have any class or series of capital stock outstanding other than the ordinary shares. As a result of the Transaction, Biota will become a wholly-owned subsidiary of Nabi. A copy of the Transaction Agreement is attached as Annex A to this proxy statement. Nabi encourages you to read the Transaction Agreement carefully and in its entirety, as it is the principal legal document that governs the Transaction.

Consideration (see page 106)

Nabi stockholders.    Nabi stockholders will continue to own their existing shares of Nabi common stock after the Transaction. Nabi stockholders should not return their stock certificates with the enclosed proxy card.

Biota stockholders.    Nabi has agreed to issue 0.669212231 shares of Nabi common stock for each ordinary share of Biota outstanding immediately prior to the completion of the Transaction, subject to certain adjustments as described below. Immediately after the completion of the Transaction, Nabi’s existing stockholders collectively will own approximately 26% of the outstanding common stock of the combined company, and Biota’s former stockholders collectively will own approximately 74% of the outstanding common stock of the combined company. In the event that Nabi conducts an issuer tender offer to purchase shares of Nabi common stock or implements the reverse stock split described in the proxy statement before the completion of the Transaction, the number of shares of Nabi common stock to be issued in exchange for each ordinary share of Biota will be adjusted in accordance with the Transaction Agreement in order to preserve the respective percentage of shares of the outstanding common stock of the combined company to be held immediately after the completion of the Transaction by Biota’s former stockholders on the one hand (collectively being approximately 74%) and Nabi’s existing stockholders on the other hand (collectively being approximately 26%).

Treatment of Equity-Based Awards (see page 106)

Biota has represented in the Transaction Agreement that, immediately prior to the completion of the Transaction, there will be no outstanding rights to acquire any ordinary shares of Biota under Biota’s equity incentive arrangements.

Opinion of Financial Advisor to the Nabi Board of Directors (see page 61)

On April 20, 2012, Houlihan Lokey Financial Advisors, Inc., which we refer to as Houlihan Lokey, rendered its written opinion to Nabi’s board of directors as to the fairness to Nabi from a financial point of view as of such date of the exchange ratio provided for in the Transaction pursuant to the Transaction Agreement.

Houlihan Lokey’s opinion was directed to Nabi’s board of directors (in its capacity as such) and only addressed the fairness to Nabi, from a financial point of view, of the exchange ratio provided for in the Transaction pursuant to the Transaction Agreement and did not address any other aspect or implication of the Transaction. The summary of Houlihan Lokey’s opinion in this proxy statement is qualified in its entirety by reference to the full text of its written opinion, which is included as Annex C to this proxy statement and sets forth the procedures followed, assumptions made, qualifications and limitations on the review undertaken and other matters considered by Houlihan Lokey in preparing its opinion. However, neither Houlihan Lokey’s opinion nor the summary of its opinion and the related analyses set forth in this proxy statement are intended to be, and do not constitute, advice or a recommendation to any Nabi stockholder as to how such stockholder should act or vote with respect to any matter relating to the Transaction.

Interests of Nabi’s Executive Officers and Directors in the Transaction (see page 67)

In considering the recommendation of Nabi’s board of directors to vote in favor of each of the proposals to be considered and voted upon at the special meeting, you should be aware that Nabi’s executive officers and directors have interests in the Transaction that may be different from, or in addition to, the interests of Nabi’s stockholders generally. Nabi’s board of directors was aware of these interests and considered them, among other matters, when it approved the Transaction Agreement and the Transaction. See “The Transaction—Interests of Nabi Executive Officers and Directors in the Transaction” beginning on page 67 of this proxy statement for more information.

Accounting Treatment (see page 71)

Under accounting principles generally accepted in the U.S. (“U.S. GAAP”), the Transaction will be accounted for as a “reverse acquisition” pursuant to which Biota will be considered the acquiring entity for accounting purposes. As such, Biota will allocate the total purchase consideration to Nabi’s tangible and identifiable intangible assets and liabilities based on their relative fair values at the date of the completion of the Transaction. Biota’s historical results of operations will replace Nabi’s historical results of operations for all periods prior to the Transaction; after completion of the Transaction, the results of operations of both companies will be included in Nabi’s consolidated financial statements.

Directors and Executive Officers Following the Transaction (see page 72)

The parties have reached certain agreements regarding the governance of the combined company following the completion of the Transaction, including the following:

Material U.S. Federal Income Tax Consequences of the Transaction (see page 76)

There are no material U.S. federal income tax consequences to Nabi’s existing stockholders that will result from the issuance of Nabi shares in the Transaction. See “Proposal 3: Amendment to Certificate of Incorporation to Effectuate Reverse Stock Split—Certain U.S. Federal Income Tax Consequences of the Reverse Stock Split” beginning on page 136 of this proxy statement for a discussion of certain U.S. federal income tax consequences of the proposed reverse stock split described in this proxy statement.

Regulatory and Other Approvals (see page 79)

Australian Regulatory Matters

Under the Corporations Act of 2001 (Cth) (“Corporations Act”), the Transaction must be approved by Biota stockholders and a Supreme or Federal Court of Australia (expected to be the Supreme Court of Victoria, Australia) to become effective. The Corporations Act expressly prevents a court from granting approval unless:

Biota intends to apply to the Supreme Court of Victoria, Australia at a first court hearing (the “first court hearing”) for (1) an order that meetings for each applicable class of Biota stockholders be convened and (2) an approval of the distribution of the explanatory memorandum about the Transaction to Biota stockholders. Biota must give ASIC at least 14 days’ notice before the first court hearing and must allow ASIC a reasonable opportunity to review the explanatory memorandum and to make submissions to the court with respect to it. Biota also intends to apply to ASIC for ASIC to provide to the court a written statement that it has no objection to the Transaction. Provided that ASIC is satisfied with the terms of the Transaction documents (including the Transaction Agreement) and the explanatory memorandum to be provided to Biota stockholders, Biota expects that ASIC will provide to the court at the first court hearing a letter stating that ASIC intends to issue a no-objection statement at the second court hearing (described below).

Pursuant to the orders made by the court at the first court hearing, Biota will convene a meeting of Biota stockholders to vote on a resolution to approve the Transaction. The Transaction requires the approval of a majority in number of Biota stockholders that are present and voting in person or by proxy at the meeting, as well as at least 75% of the votes cast on the resolution in person or by proxy by Biota stockholders at the meeting. The Biota stockholders meeting is expected to occur on or about [****], 2012.

If the Transaction is approved at the Biota stockholders meeting and all other conditions to the Transaction are satisfied, Biota will seek to obtain court approval of the Transaction at the second court hearing (the “second court hearing”). The second court hearing is expected to occur on or about [****], 2012. If the court approves the Transaction, a copy of the court order will be filed with ASIC and the Transaction will become binding on all applicable classes of Biota stockholders, including those who voted against the Transaction (the “effective date of the scheme of arrangement”).

Trading in ordinary shares of Biota on the ASX will be suspended from the close of trading on the effective date of the scheme of arrangement, which is anticipated to be shortly after the date of court approval. A record date (which will be the fifth business day following the effective date of the arrangement) will be set to determine the Biota stockholders entitled to receive shares of Nabi common stock as consideration in the Transaction. The Transaction consideration will be provided to Biota stockholders three business days after such

record date and the Transaction will be deemed to have been completed or implemented on that date (the “implementation date”).

Biota Stockholder Approval

Biota will hold a special meeting of its stockholders on or about [****], 2012 to consider and vote upon a resolution to approve the Transaction. The Transaction must be approved by the requisite majorities of the Biota stockholders under Section 411(4)(a)(ii) of the Corporations Act (a majority in number of Biota stockholders that are present and voting in person or by proxy at the meeting, as well as at least 75% of the votes cast on the resolution in person or by proxy by Biota stockholders at the meeting).

HSR Approval

The Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, and the implementing regulations thereunder (the “HSR Act”) require the parties to certain acquisitions and other transactions that meet specified minimum size requirements to file notifications with the Antitrust Division of the U.S. Department of Justice and the U.S. Federal Trade Commission and observe a waiting period before closing, unless an exemption applies. Nabi and Biota believe that the Transaction is exempt from the notification requirements based on the fair market value of Biota’s non-exempt assets, namely its assets in the U.S. Accordingly, neither Nabi nor Biota is expected to make a notification under the HSR Act with respect to the Transaction. If such exemption does not apply, the Transaction would not be able to be consummated until notification and report forms have been filed with the Antitrust Division of the U.S. Department of Justice and the Federal Trade Commission by Nabi and Biota, and the applicable waiting period has expired or been terminated.

Other than as described above, neither Nabi nor Biota is aware of any regulatory approvals required to be obtained, or waiting periods required to expire, to complete the Transaction. If Nabi and Biota discover that other approvals or waiting periods are necessary, Nabi and Biota intend to seek to obtain or comply with them in accordance with the Transaction Agreement.

Listing of Nabi Common Stock (see page 80)

Nabi has agreed to obtain listing approval from the NASDAQ for the Nabi shares that will be issued in the Transaction.

No Appraisal Rights (see page 76)

Under Delaware law, holders of shares of Nabi common stock are not entitled to appraisal rights in connection with the Transaction or any of the matters to be acted on at the special meeting.

Conditions to Completion of the Transaction (see page 108)

As more fully described in this proxy statement and in the Transaction Agreement, the completion of the Transaction is conditioned on the satisfaction or, where legally permissible, waiver of a number of conditions, including among others:

Each party’s obligation to complete the Transaction is subject to certain other conditions, including the absence of any injunction, restraint or governmental restriction prohibiting the Transaction, the accuracy of the other party’s representations and warranties contained in the Transaction Agreement, material compliance by the other party with its obligations under the Transaction Agreement, and the absence of a material adverse effect related to the other party. Nabi cannot be certain when, or if, the conditions to the Transaction will be satisfied or waived, or that the Transaction will be completed. Approval, on an advisory (non-binding) basis, of the compensation payable under existing arrangements to certain executive officers of Nabi and approval of the proposed amendment to the Nabi certificate of incorporation to effect a reverse stock split are not conditions to completion of the Transaction.

Non-Solicitation of Other Offers (see page 122)

The Transaction Agreement obliges each party to abide by certain restrictions on such party’s ability to solicit competing proposals from third parties and to provide non-public information to and enter into discussions or negotiations with third parties regarding competing proposals. Notwithstanding this obligation, each party may under certain circumstances furnish information to and engage in discussions or negotiations with third parties with respect to unsolicited competing proposals if such company’s board of directors determines in good faith that the competing proposal is, or may reasonably be expected to lead to, a superior proposal, as defined in the Transaction Agreement, and that not taking such action would, or would be likely to, involve a breach of the fiduciary or statutory duties owed by a director.

Termination of the Transaction Agreement (see page 123)

The Transaction Agreement may be terminated and the Transaction abandoned at any time before the effective time of the Transaction upon the mutual agreement of Nabi and Biota. In addition, either Nabi or Biota may terminate the Transaction Agreement if:

Nabi may terminate the Transaction Agreement if Biota’s board of directors publicly changes (including by attaching qualifications) or withdraws its recommendation in favor of the Transaction or publicly recommends, promotes or otherwise endorses a superior proposal. Similarly, the Transaction Agreement may be terminated by Biota if (1) Nabi’s board of directors publicly changes (including by attaching qualifications) or withdraws its recommendation in favor of the Transaction Proposals or publicly recommends, promotes or otherwise endorses a superior proposal or (2) if Nabi does not comply with its obligations, after the Transaction is approved by the Supreme Court of Victoria, Australia, to provide the Nabi closing net cash balance certificate to Biota on the implementation date showing a Nabi closing net cash balance of no less than $54 million.

Expenses and Break Fees (see page 124)

In certain circumstances described under “The Transaction Agreement—Break Fee” beginning on page 124 of this proxy statement, Nabi may be obligated to pay Biota a break fee of A$2.0 million or Biota may be obligated to pay Nabi a break fee of A$2.0 million. The Transaction Agreement generally provides that each party will bear its own costs and expenses, except as described under “The Transaction Agreement—Costs and Expenses” beginning on page 126 of this proxy statement.

Repurchase of Shares of Nabi Common Stock; Dividends and Distributions (see page 77)

Nabi plans to return to its stockholders, before the completion of the Transaction, its remaining cash in excess of the $54 million required to be held by Nabi at the completion of the Transaction after satisfying outstanding liabilities. Such distribution is expected to take the form of a repurchase of certain of the outstanding shares of Nabi common stock through an issuer tender offer and, for any remaining amounts, through a dividend or return of capital, and currently is expected to be in the range of approximately $25 million to $30 million in the aggregate. Nabi plans to commence an issuer tender offer to repurchase certain of the outstanding shares of Nabi common stock after filing a preliminary form of this proxy statement with the SEC and to complete such issuer tender offer before mailing a definitive form of this proxy statement to Nabi stockholders. After the completion of the issuer tender offer but prior to the completion of the Transaction, Nabi plans to declare a dividend or effect a return of capital, as applicable, to distribute any remaining cash in excess of the $54 million required to be held by Nabi at the completion of the Transaction that is not required to satisfy any outstanding liabilities.

Contingent Value Rights (see page 77)

Nabi plans to issue contingent value rights (each, a “CVR” and collectively, the “CVRs”) to existing Nabi stockholders before the completion of the Transaction. Nabi expects that one CVR will be issued for each share of Nabi common stock outstanding as of a record date to be set at a date prior to the completion of the Transaction. However, the CVRs will not be attached to the shares of Nabi common stock. A CVR will be a non-transferrable (subject to certain limited exceptions) right to potentially receive certain cash payments in connection with a NicVAX Transaction (as defined in the CVR Agreement) upon the terms and subject to the conditions set forth in a Contingent Value Rights Agreement to be entered into between Nabi and a rights agent (the “CVR Agreement”). The form of the CVR Agreement is attached to this proxy statement as Annex G.

See “Repurchase of Shares of Nabi Common Stock; Dividends and Distributions—Contingent Value Rights” beginning on page 77 of this proxy statement for more information regarding the CVRs, the CVR Agreement and the NicVAX Transaction.

Questions

If you have additional questions about the Transaction or other matters discussed in this proxy statement after reading this proxy statement, you should contact Morrow & Co., LLC, Nabi’s proxy solicitation agent. The address of Morrow & Co., LLC is 470 West Avenue, Stamford, CT 06902. You can call Morrow & Co., LLC at (203) 658-9400 or toll-free at (800) 662-5200.