FORM 6-K

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                        Report of Foreign Private Issuer

                    Pursuant to Rule 13s - 16 or 15d - 16 of
                      the Securities Exchange Act of 1934

                            For the month of May 2006


                                  Acambis plc
                (Translation of registrant's name into English)

                           Peterhouse Technology Park
                               100 Fulbourn Road
                               Cambridge CB1 9PT
                                    England

                    (address of principal executive offices)

    (Indicate by check mark whether the registrant files or will file annual
                 reports under cover of Form 20-F or Form 40-F

                         Forms 20-F X     Form 40-F

  Indicate by check mark whether the registrant by furnishing the information
     contained in this Form also thereby furnishing the information to the
                Commission pursuant to Rule 12g3-2(b) under the
                       Securities Exchange Act of 1934).

                              Yes       No X

 (if "Yes" is marked, indicate below the file number assigned to the registrant
                   in connection with Rule 12g3-2(b): 82- ).




Enclosure:

'Opposition to MVA Patent'


Acambis and Baxter file oppositions to European MVA patent


Cambridge, UK and Cambridge, Massachusetts - 19 May 2006 - Acambis plc (Acambis)
(LSE: ACM, NASDAQ: ACAM) announces that it has filed an opposition to European
Patent No. 1 335 987 issued to Bavarian Nordic A/S (Bavarian Nordic) on 28
December 2005 by the European Patent Office.

The patent relates to Bavarian  Nordic's MVA  technology,  MVA-BN(R).  MVA is an
attenuated  smallpox vaccine,  Modified  Vaccinia Ankara,  that has been used in
Europe since the late 1970s.  Acambis opposes the patent on the grounds that the
patent is invalid.  Acambis' partner,  Baxter Healthcare SA, which  manufactures
Acambis' MVA vaccine,  MVA3000, in Austria,  has also filed an opposition to the
European patent on the grounds that the patent is invalid.

Acambis is  bidding  for a US  Government  contract  to supply an  emergency-use
stockpile of MVA.

Gordon Cameron, Chief Executive Officer of Acambis, commented:

"We  strongly  believe that the patent  awarded to Bavarian  Nordic in Europe is
invalid.  This belief is  supported by evidence  presented at the  International
Trade  Commission  hearing  held over the last two weeks in relation to Bavarian
Nordic's US patent for MVA-BN(R). Filing this opposition is part of our strategy
vigorously  to defend our  freedom  to  operate in the MVA field,  which we will
defend in whatever forum necessary."


                                     -ends-




Enquiries:


Acambis plc

                                                            

Gordon Cameron, Chief Executive Officer:    Tel +1 (617) 761 4200

David Lawrence, Chief Financial Officer:    Tel: +44 (0) 1223 275 300

Lyndsay Wright, VP, Communications and IR:  Tel +44 (0) 1223 275 300


Financial Dynamics

David Yates/Anna Keeble:                    Tel +44 (0) 20 7831 3113



About Acambis

Acambis is a leading  developer  of  vaccines  to prevent  and treat  infectious
diseases.  Recognised  internationally  as  the  leading  producer  of  smallpox
vaccines,  Acambis is developing an investigational smallpox vaccine,  ACAM2000,
and is manufacturing  emergency-use  stockpiles of this investigational  vaccine
for the US  Government  and  other  governments  around  the  world.  It is also
developing an attenuated smallpox vaccine,  MVA3000, under contracts with the US
National  Institutes of Health.  Acambis'  US-based  subsidiary  Berna  Products
Corporation markets Vivotif(R),  the world's only licensed oral typhoid vaccine,
in  North   America.   Acambis'   investigational   vaccine   against   Japanese
encephalitis, ChimeriVax-JE, is undergoing Phase 3 clinical testing. It also has
the most advanced investigational vaccine against the West Nile virus, which has
spread  to 48 US  States  in  the  last  seven  years,  and  a  vaccine  against
Clostridium difficile bacteria, a leading cause of hospital-acquired infections.


Acambis is based in Cambridge, UK and Cambridge,  Massachusetts, US. Its primary
listing is on the London Stock  Exchange  (ACM) and its shares are listed in the
form of American  Depositary  Receipts on NASDAQ  (ACAM).  More  information  is
available at www.acambis.com.


"Safe Harbor"  statement under the Private  Securities  Litigation Reform Act of
1995:

The  statements  in  this  news  release  that  are  not  historical  facts  are
forward-looking  statements that involve risks and uncertainties,  including the
timing and results of clinical trials,  product  development,  manufacturing and
commercialisation risks, the risks of satisfying the regulatory approval process
in a timely manner, the need for and the availability of additional capital. For
a discussion of these and other risks and uncertainties see "Risk management" in
the Company's  Annual Report and "Risk factors" in its Form 20-F, in addition to
those detailed on the Company's  website and in the Company's  filings made with
the Securities and Exchange Commission from time to time. These  forward-looking
statements  are based on estimates  and  assumptions  made by the  management of
Acambis and are believed to be reasonable,  though are inherently  uncertain and
difficult to predict.  Actual results or experience could differ materially from
the forward-looking statements.

                                      SIGNATURE

Pursuant  to the  requirements  of the  Securities  Exchange  Act of  1934,  the
registrant  Peptide  Therapeutics Group has duly caused this report to be signed
on its behalf by the undersigned, thereunto duly authorized.



Date: 19 May 2006                      ACAMBIS PLC





                                        By:  /s/ Lyndsay Wright
                                             Name: Lyndsay Wright
                                             Title: VP, Communications and IR.